The present invention relates to, the process of making silicon-nitride (Si.sub.3 N.sub.4) composites and the resultant products utilizing the polymeric ceramic precursors disclosed in the parent application set forth above, whose entire specification and claims are specifically incorporated herein by reference.
Polymer-matrix composites, of which Fiberglas is probably the most familiar, enjoy widespread use in the aerospace industry, and growing use in the automotive industry. While their convenience of fabrication often outweighs their disadvantages, the limitations of polymer-matrix composites have fostered the development of metal-matrix and, more recently, ceramic-matrix composites. Excellent high-temperature strength retention and resistance to oxidation give ceramics the potential to replace metals in many structural applications, including airframes and engines. The high strength to density ratio of ceramic components helps to decrease a system's total weight, and thus to increase its operating efficiency.
However, while ceramics have enormous potential, their adoption has been seriously hampered by their brittle fracture behavior, high flaw sensitivity, and resulting low tensile strength. Incorporation of continuous fibers into a ceramic matrix may yield a composite material with superior properties by providing a toughening mechanism that lessens the propensity for catastrophic failure.
There are a number of techniques which have been used to prepare ceramic composites, all of which have inherent difficulties. One technique is the use of ceramic powder processing techniques. This involves the consolidation of micron-sized particles through sintering and treatment at a high temperature (usually above 1300.degree. C.). Unlike metals which are somewhat compliant, ceramic particles are very hard and abrasive and the fibers that are utilized to reinforce such composites are often broken when such methods are employed. The fiber breakage is very detrimental to the composites integrity so that the resultant composite often exhibits mechanical properties inferior to that of the unreinforced composite.
Moreover, in utilizing ceramic powders a green body is first formed, as by slip molding, injection molding, extrusion, or cold or hot pressing, followed by sintering of the green body. Sintering most often results in products that contain voids or other defects and lacking the accurate dimensions required, thus necessitating subsequent expensive machining.
Another technique is the "sol-gel" technique of ceramic fabrication in which inorganic polymers are decomposed into ceramics when heated. While sol-gel processing has been used to fabricate fiber-reinforced composites, the ceramic yields are very low and so a large number (220) infiltration cycles are required to obtain a part with an acceptable level of porosity. In addition, current sol-gel practice is limited to oxide ceramics.
A further technique is that of reaction bonding. This technique generally has been applied to non-oxide ceramics such as silicon carbide, silicon nitride, boron nitride, and the like, but has the disadvantages of long reaction time at elevated temperature, limited net shape capability, and the potential for fiber degradation among others.